Many attempts have been made to improve the efficiency of glass melting furnaces over the past 50-75 years. To date, however, very few new melting concepts have been adopted by the glass industry.
These attempts to increase glass melting/glass production efficiency have included, in particular, glass batch material preheating techniques used in conjunction with different glass melting methods to increase the rate at which the glass melting process occurs. Batch material preheating processes which have been tried include: using moving bed reactors, raining bed reactors, fluidized bed reactors, counter-flow suspension reactors, plug flow type suspension reactors and dump combustors with flame holders to preheat the batch material prior to melting. In addition to attempts to increase efficiency by preheating the batch material, enhanced or improved glass melting processes have included methods utilizing submerged combustion, direct heating of moving batch surfaces, melting over bodies of revolution or other surfaces, melting in rotating cylinders and melting in cyclone type reactors.
Of particular interest with respect to the present invention is glass melting in cyclone type reactors. Previously, patents relating to cyclone type reactors have issued to Ferguson, U.S. Pat. No. 2,006,947; Jack, et al., U.S. Pat. No. 3,077,094; Boivent, U.S. Pat. Nos. 3,443,921 and 3,510,289; Ito, U.S. Pat. No. 3,784,113; Niefyodon et al., U.S.S.R. Pat. No. 0708129 and Hnat, U.S. Pat. No. 4,553,997 and 4,544,394. Each of these patents discloses the use of a cyclone reactor for the final glass melting step and includes combustion or other forms of heat addition, such as penetrating burners within the cyclone melting chamber, in order to elevate the batch materials to the requisite glass melting temperature.
The previous patents to Hnat pay particular attention to the use of various types of cyclone designs as well as the use of specific, separate batch injection locations as means of controlling the losses of higher volatile mineral matter, such as soda ash and borox. Otherwise, the prior cyclone melting approaches for improving glass production efficiency have generally not considered means of limiting the losses of volatile mineral matter such as fluxing agents, viscosity control agents, fining agents or reducing agents prior to melting in the cyclone reactor. In particular, methods of controlling the time-temperature history of the volatile mineral matter in the suspension preheating steps have not been previously developed.
Prior glass melting methods utilizing ash containing fuels, such as coal, have not been successful because of the poor economics associated with coal gasification processes or, as in the case of direct coal firing, because the ash contamination in the glass has been unacceptable from the standpoint of quality control. Even though typical coal ashes have constituent species which are identical to thoses found in commercial glasses, the concentration distribution of the individual constituent species is substantially different. The iron oxide concentrations in coal ashes are typically much higher than concentrations found in common commercial glasses. Coal ashes typically have iron oxide concentrations in the range of 10-20%, whereas most glass compositions have iron oxide concentrations of less than 0.1-0.2%, and iron oxide concentrations for flint container glass must be generally lower than 0.02% if acceptable coloration is to be achieved. The quality control requirements for amber and green bottle glass are less restrictive, but the quality control requirements still generally require that iron oxide concentrations be less than 0.1 and 0.3%, respectively.
With insulation fiberglass, higher levels of iron oxide are tolerable, with iron oxide concentrations of 1-2% being acceptable. Iron oxide levels higher than 1-2% generally lead to a degradation of the insulating value and can cause material compatibility problems with existing fiberizers. Mineral wools, which are often made from blast furnace slags, have iron oxide concentrations in the same range as coal ash; therefore, the production of this product is not very sensitive to coal ash contamination. The efficiency of mineral wool production, however, is substantially less than the production of insulation fiberglass because of the previously mentioned material compatibility problems with high efficiency fiberizers.
Because of the ash contamination problems, and in particular the problem of iron oxide contamination, very few prior glass melting inventions have considered or succeeded in direct firing using coal or other fuels containing substantial amounts of ash as a fuel. In fact, direct firing of conventional open hearth-type furnaces with pulverized coal has been unsuccessful because of ash carry over into the regenerators. Furthermore, refractory corrosion and blockage problems have occurred, as well as the formation of stones and cords within the melt, because of slagging with the furnace chamber.
The abiltiy to fire the glass melting systems with fuels subject to ash contamination is now an important consideration in light of the fuel efficiency and the high temperature heating that can be obtained, but use of these fuels has not, heretofore, been successfully achieved. In the recent prior art of Demarest et al., U.S. Pat. No. 4,634,461, the possibility of using pulverized coal in a rapid glass melting process is taught; however, in that patent the coal ash is actually incorporated into the glass batch materials and the final glass product with no means of controlling the level of ash contamination.